Energy-efficiency in wireless communications networks, e.g. a Long Term Evolution (LTE) network, is important for many reasons. The energy consumption is a major operating cost for network operators and therefore energy-saving features are of high interest. Operators' energy operating expenses (OPEX) are expected to continue to increase and the energy performance of network nodes such as evolved NodeBs (eNodeBs) as well as other entities of the network may be a sales advantage.
Some results from energy consumption studies in mobile networks have shown that a non-negligible part of the network operator's energy consumption in their wireless networks comes from physical layer processing and transmission. This can be justified in scenarios where there is always traffic in a given area. However, due to the LTE design there is a high and almost constant energy consumption even when there is not traffic in a given cell.
FIG. 1 shows the energy consumption for one LTE network in Europe for the following scenarios:                Scenario 1: “the most relevant traffic scenario for 2015”        Scenario 2: “an upper bound on the anticipated traffic for 2015”        Scenario 3: “an extremity for very high data usage in future networks”        
The left-hand side diagram of FIG. 1 shows ratio of empty subframes for the different scenarios. When there is no traffic, the ratio is 100%, but the ratio of empty subframes is high or very high also for the above scenarios (95%, 90% and 81%, respectively). The right-hand side diagram of FIG. 1 illustrates the energy consumption (area power) for the above scenarios. There is a fixed energy consumption and a dynamic energy consumption depending on the amount of traffic. As can be seen from the results shown in FIG. 1, there is a high consumption even for a cell with little traffic. A large part of the energy comes from the fixed energy consumption, for instance comprising constantly transmitted cell specific reference signals (CRSs), broadcasted over the whole bandwidth. The way system information (SI) is acquired in LTE represents a non-negligible amount of signals constantly broadcasted over the air interface.
Before a communication device, in the following denoted user equipment (UE), can access a wireless communications network, it has to acquire the system information. This is done in different ways depending on whether the UE is roaming, recovering from radio link failure (RLF) or powering on. However, some general steps are typically similar.
At the network side, a certain amount of information is broadcast in each cell. The first information is a primary synchronization signal (PSS) and a secondary synchronization signal (SSS), which are used by the UE to obtain frequency and time (symbol and frame) synchronization. These signals also encode the physical cell identity (PCI). After this physical layer synchronization and PCI detection the UE is capable of performing channel estimation by using cell specific reference signals (CRSs), which are constantly broadcasted, and finally decode the system information in a few steps.
From the physical layer point of view, PSS/SSS and CRSs are always broadcasted by the network. These are used by the UE for synchronization and for being able to perform channel estimation.
System information in LTE is structured by means of System Information Blocks (SIBs), each of which comprises a set of functionally-related parameters. The SIB types that have been defined include:                Master Information Block (MIB) comprises a limited number of the most frequently transmitted parameters which are essential for a UE's initial access to the network.        System Information Block Type 1 (SIB1) comprises parameters needed to determine if a cell is suitable for cell selection, as well as information about the time-domain scheduling of other SIBs.        System Information Block Type 2 (SIB2) comprises common and shared channel information.        SIB3, SIB4, SIB5, SIB6, SIB7, SIB8 comprise parameters used to control intra-frequency, inter-frequency and inter-RAT cell reselection.        SIB9 is used to signal the name of a Home evolved NodeB (HeNB), HeNB being a LTE specific term denoting a low-power smart cell.        SIB10, SIB 11, SIB12 comprises the Earthquake and Tsunami Warning Service (ETWS) notifications and Commercial Mobile Alert System (CMAS) warning messages.        SIB13 comprises Multimedia Broadcast Multicast Services (MBMS) related control information.        SIB14 is used to configure Extended Access Class Barring.        SIB15 is used for convey MBMS mobility related information.        SIB16 is used to convey Global Positioning System (GPS)-related information.        
This list of System Information Block Types has been expanding over the years and it is expected to continue to increase during next-coming Third Generation Partnership Project (3GPP) releases.
3GPP defines as “essential information”, the information contained in MIB, SIB Type 1, and SIB Type 2. For UEs that are Extended Access Barring (EAB) capable, the information in SIB Type 14 is also considered to be essential information. “Essential information” means that the UE needs to acquire the information prior to accessing the network.
The system information is constantly broadcasted, but depending on the type of information, different periodicities are assumed. In LTE the time-domain scheduling of the MIB and SIB1 messages is fixed with periodicities of 40 ms and 80 ms. Furthermore, for the MIB the transmission is repeated four times during each period, i.e. once every 10 ms. SIB1 is also repeated four times within its 80 ms period, i.e. every 20 ms, but with different redundancy version for each transmission.
The time-domain scheduling of the SI messages (for the other SIBs) is dynamically flexible: each SI message is transmitted in a defined periodically-occurring time-domain window, while physical layer control signaling indicates in which subframes within this window the SI is actually scheduled. The scheduling windows of the different SI messages (referred to as SI-windows) are consecutive, i.e. there are neither overlaps nor gaps between them, and they have a common length that is configurable. SI-windows can include subframes in which it is not possible to transmit SI messages, such as subframes used for SIB1 and subframes used for the uplink in Time division duplex (TDD).
FIG. 2 illustrates an example, for the case of LTE, of the time-domain scheduling of SI, showing the subframes used to transfer the MIB (black squares in FIG. 2), SIB1 (indicated by crosses) and four SI messages. The example uses an SI window of length 10 subframes, and four such SI windows are shown, numbered from 1 to 4. The first SI-window (SI-window 1) is a radio frame with System Frame Number (SFN)=0, and a MIB is included in the first subframe thereof, and in the second, fourth and seventh subframes other SI messages are included. In the sixth subframe SIB 1 is included. As can be seen, the SI may be scheduled differently within the different SI-windows.
Energy-saving features are important also for the communication device, e.g. user equipment (UE). One known feature for saving battery operation time is a Discontinuous Reception (DRX) functionality. In LTE, the DRX functionality can be configured for UEs in both the RRC_IDLE state (Radio Resource Control, RRC) and RRC_CONNECTED state.
For DRX in RRC_CONNECTED state a DRX cycle comprises an active (“on”) part and a passive (“sleep”/“off”) part. During the active part, i.e. the ‘On Duration’, the UE should monitor downlink channels such as Physical Downlink Control Channel (PDCCH), monitor downlink signals such as Primary Synchronization Signal/Secondary Synchronization Signal (PSS/SSS) and CRSs in order to perform measurements, and decode messages such as system information blocks.
During the sleep period, comprising the remainder of the DRX cycle, the UE is configured to skip the monitoring actions for battery saving purposes and can thus not be reached for downlink transmissions for the duration of this sleep period. The parameterization (e.g. length of the sleep period) of the DRX cycle involves a tradeoff between battery saving and latency. On the one hand, a long DRX period is beneficial for lengthening the UE's battery life. On the other hand, in the case of the UE being in RRC_IDLE state, long periods can delay UE responses to paging and for UEs in RRC_CONNECTED state a long DRX cycle may delay delivery of downlink data. The active period has a configured minimum length controlled by an on-duration timer of the UE, which can be dynamically extended if downlink activity occurs during the active period.
FIG. 3 shows an example of a DRX configuration for a UE in RRC_CONNECTED state. When a scheduling message is received during an ‘On Duration’ (active part), the UE starts a ‘DRX Inactivity Timer’ and monitors the downlink channels in every subframe while the DRX Inactivity Timer is running. During this period, the UE can be regarded as being in a continuous reception mode. Whenever a scheduling or paging message is received (indicated by “MAC CE reception” in the FIG. 3, for Media Access Control Control Element) while the DRX Inactivity Timer is running, the UE restarts the DRX Inactivity Timer, and when it expires the UE moves into a short DRX cycle and starts a ‘DRX Short Cycle Timer’. The UE is free to send uplink data at any time during a DRX cycle, i.e. during the active part as well as during the passive part.
In LTE, DRX configuration for a UE in RRC_CONNECTED state is provided to the UE via dedicated RRC signaling. The network, e.g. an evolved NB (eNB) thereof, may also dynamically impact the UE's DRX behavior via Media Access Control (MAC) signaling.
When using DRX in RRC_IDLE state in LTE, a UE monitors the relevant paging occasions for paging messages intended for the UE, but can remain in a low-power sleep state between these paging occasions. The paging occasions that are applicable for a UE are derived by both the eNB and the UE from a combination of system information parameters and UE specific parameters. Further, the UE may also measure on signals in order to perform cell reselection.
The current solutions for system access in LTE attempts to reduce the time to access the system in all these different scenarios, e.g. when the UE has no prior information about the system, such as when powering on or roaming. A major drawback of such solutions is the high energy consumption or waste due to the constantly broadcasted reference signals and information, especially in the case where there are many cells without traffic during certain periods.
This unnecessary energy consumption and also potentially generated interference to UEs in other cells in these traffic scenarios comes from the fact that system access in LTE depends on broadcasted information over the air. The system access in LTE depends, for instance, on broadcasted information such as PSS/SSS for physical layer synchronization and PCI detection, the MIB, SIB1 and SIB2 (wherein about 1000 bits over the air within a repeated window of few hundred milliseconds) and CRSs at least within the bands of MIB, SIB1 and SIB2.
The time needed to access the system can be kept short for UEs and other devices that want to access the otherwise empty cell without having any prior information about the cell; however this is not an energy efficient solution.